1. Field of the Invention
The present invention is related generally to electromechanical actuation systems and, more particularly, to a multi-quadrant brushless motor drive system for controlling the magnitude and direction of the torque output of a DC motor.
2. Description of the Prior Art
Numerous applications require an actuation system for positioning a specified load as a function of a position command signal. Actuation systems used to control aircraft flight are of particular importance in the design of safe, energy-efficient aircraft. The design of an electromechanical actuation system or an aircraft flight control system is complicated by the necessity of satisfying numerous, and often conflicting, requirements such as steady-state and dynamic performance, duty cycle, weight, envelope, installation, reliability and cost.
A variety of motor control systems are known, none of which completely satisfies the needs met by arrangements in accordance with the present invention. For example, the Masse et al U.S. Pat. No. 3,683,254 discloses a servo system which contains provision for protecting against instabilities and system performance due to system noise. The system provides for automatic noise cancellation and acts, through velocity control in the servo system driving circuitry, to limit the drive whenever the system is subjected to noise. However, the disclosed servo system is directed to the control of a stepping motor, and therefore is not pertinent to the present system for controlling a brushless DC motor.
The Dixon et al U.S. Pat. No. 4,019,107 discloses an arrangement for controlling a shunt DC motor, rather than the permanent magnet DC motor to which the present system is directed. Moreover, this system utilizes a tachometer readout to develop a motor speed signal which is required in the control system.
Imamura U.S. Pat. No. 4,156,169 discloses a system designed to control the velocity of a motor by comparing a velocity signal with a velocity command signal to vary the motor drive. This differs from the present system which is capable of developing control without resort to motor shaft velocity.
The Liska et al U.S. Pat. No. 4,167,693 discloses a commutation circuit for controlling a brushless DC motor. However, the control circuitry of this patent is principally directed to reducing the sensitivity of the drive circuits to self-induced current spikes.
Smooth motor operation is essential in providing the degree of safety and control required in aircraft actuator systems. Such prior art DC motor systems as are known fail to provide sufficiently smooth motor operation for electromechanical actuators used in aircraft flight control systems. A typical prior art DC motor drive system controls the operation of the motor only in the first quadrant, where both the voltage and current are positive, and in the third quadrant where both the current and voltage are negative. A DC motor drive system which provides driven operation of a motor only in the first and third quadrants ignores the transients which some switching actions develop in the second and fourth quadrants. The failure of prior art systems to provide smooth motor operation during voltage and current transients throughout the operating cycle results from discontinuities in the logic which controls the switching actions. The difficulties associated with the prior art motor drive circuits are particularly acute in electromechanical actuators such as may be used in aircraft flight control systems because the motors in such systems are often operated at low speed where the effects of logic discontinuities have the greatest effect.
There are prior art DC motor drive systems which provide four-quadrant operation; however such systems generally require an input signal indicative of the direction of rotation of the rotor. Known prior art four-quadrant motor drive systems are complex and fail to provide the smoothness of motor operation required for aircraft flight control systems.
A particular DC motor control system is needed which can provide smooth, precisely controllable motor operation for aircraft flight control systems which are suitable for replacing hydraulic actuation systems presently in use.